If it was rough sex or even not - it could've agitated the uterine lining and caused it to bleed.
However... I don't know how much you're bleeding or anything, so i honestly couldn't tell you.
If it's light and kind of pinkish/reddish, then.. that's probably the case.
If it's heavier and more red - I would just keep an eye out.. maybe see if it gets heavier, any pain occurs, and/or if it's longer than a few days.
However, it could be a late/early period as well, and the pleasure of the sex masked the cramping. :]
at 3:20 AM on Sep. 16, 2009